Method of texturing magnetic hard disk substrate

ABSTRACT

Texturing marks are formed on the surface of a substrate of a magnetic hard disk first by forming approximately concentric circular preliminary marks in a first step and then forming in a second step approximately concentric circular texturing marks on the surface of the substrate based on the preliminary marks formed in the first step. The surface after the texturing marks are formed in the second step has average surface roughness in the range of 1 Å or more and 6 Å or less and a ratio of maximum surface roughness to the average surface roughness in the range of less than 10. A foamed tape and a lubricant not containing any abrading particles are used in the second step. The foamed tape has a foamed layer having average diameter of air bubbles in the range of 1 μm or more and 50 μm or less, compressibility in the range of 3% or more and 7% or less, compression recovery ratio in the range of 40% or more and 60% or less, Shore D hardness in the range of 20 degrees or more and 30 degrees or less, and thickness in the range of 50 μm or more and 80 μm or less.

This application claims priority on Japanese Patent Application2005-299521 filed Oct. 14, 2005.

BACKGROUND OF THE INVENTION

This invention relates to a texturing method for forming approximatelyconcentric circular texturing marks on the surface of a magnetic harddisk substrate.

Magnetic hard disks are being used as a medium for recording data suchas sound and image for data recording and reproducing apparatus such ascomputers. A magnetic hard disk is generally produced bymirror-polishing the surface of a non-magnetic substrate such as a glasssubstrate or an aluminum substrate with Ni—P plating, carrying out atexturing process on its surface to form approximately concentriccircular texturing marks thereon and sequentially forming a magneticlayer, a protective layer, etc. on this textured surface by using aknown thin-film technology such as sputtering.

As known to persons skilled in the art, the texturing process is forforming approximately concentric circular line marks on the surface of amagnetic hard disk similar to the texturing marks formed on the surfaceof the substrate in order to prevent the adsorption of the magnetic headto the magnetic hard disk.

For the purpose of carrying out magnetization of a magnetic hard diskaccurately for accurate recording and reproduction, the surface of themagnetic hard disk must satisfy the following four topologicalconditions.

(1) Firstly, the pitch of the line marks formed on the surface of themagnetic hard disk must be made smaller. In other words, if the numberof line marks per unit length in the radial direction of the disk isincreased, the number of protruding parts of the linear marks per unitarea facing the magnetic head (or the surface portions of the magnetichard disk near the magnetic head) increases such that it becomespossible to carry out the magnetization of the magnetic hard disk moreaccurately. In recent years, the number of line marks per unit length inthe radial direction of the disk, or the line density, is coming to berequired to be 40 lines/μm or more.

(2) Secondly, deep indentations (deep indentations of line marks andscratches) must not be formed on the surface of the magnetic hard disk.This is because, if these indented parts are too deep, magnetic fluxfrom the magnetic head does not reach the magnetic layer near the bottomof the indented parts and cannot magnetize these parts. This makesaccurate recording and reproduction impossible. It is also because amagnetic layer may fail to be formed near the bottom of the indentedparts at the time of forming a thin film by sputtering.

(3) Thirdly, abnormal protrusions reaching the floating distance of themagnetic head must not be formed such that the magnetic head can fly ata low height. This is because, if the magnetic head collides with suchprotrusions, the magnetic head may be damaged and the pieces of theprotrusions will become attached to the surface of the magnetic harddisk such that accurate recording to and reproduction from the magnetichard disk become impossible. In recent years, floating distances of 10nm or less are being required.

(4) Fourthly, the surface roughness of the magnetic hard disk must bemade low such that the magnetic head can slide smoothly on the surfaceof the magnetic hard disk after landing thereon and before floating uptherefrom.

In summary, it is required to form line marks having indentations withan appropriate depth and protrusions with an appropriate height, andsuch topological surface conditions of a magnetic hard disk dependslargely on the texturing process carried out on the surface of itssubstrate.

As described in Japanese Patent Publication Tokkai 2005-131711,texturing is conventionally carried out by supplying slurry havingabrading particles dispersed therein to the surface of the substrate andpressing a tape onto the surface of the substrate and it is becomingpossible to form texturing marks with a small pitch without formingabnormally high protrusions by correctly selecting the kind and size ofthe abrading particles and the kind of the tape.

With such prior art technologies, however, abrading particles andpolishing debris that remain on the substrate surface after thetexturing process are removed only by blowing a washing liquid onto thesubstrate surface. Thus, the magnetic layer and the protective layer arenow being formed on a textured surface under such conditions thatabnormally deep indentations and scratches may be left, the surfaceroughness may not be reduced to the level for allowing the magnetic headto slide smoothly on the surface or be adjusted to the level forpreventing the adsorption of the magnetic head to the surface. FIG. 4 isa computer-generated image of a substrate surface after a prior arttexturing process.

In view of the above, there is a demand for forming texturing line marksalso on the surface of a substrate having indentations with anappropriate depth and protrusions with an appropriate height in order tosatisfy the aforementioned topological conditions required of thesurface of a magnetic hard disk. As one of judgment standards in thetechnological field of texturing, the magnitude of the ratio of themaximum surface roughness (Rmax) with respect to the average surfaceroughness in the peripheral direction of the substrate (Ra) (or theaverage value of the height difference of unevenness formed on thesurface) is coming to be considered on the textured surface in recentyears. In recent years, the value of this ratio Rmax/Ra is required tobe less than 10. It is also being required that the average surfaceroughness Ra of the substrate be 1 Å or more and 6 Å or less.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a texturingmethod capable of forming texturing marks having indentations with anappropriate depth and protrusions with an appropriate height on thesurface of a magnetic hard disk substrate.

The present invention therefore relates to a texturing method forforming texturing marks on the surface of a substrate of a magnetic harddisk, and the method of this invention is characterized as comprising afirst step of forming approximately concentric circular preliminarymarks on the surface of the substrate and a second step of formingapproximately concentric circular texturing marks on the surface of thesubstrate based on the preliminary marks formed in the first step. Thesurface after the texturing marks are formed in the second step ischaracterized as having average surface roughness in the range of 1 Å ormore and 6 Å or less and a ratio of maximum surface roughness to theaverage surface roughness in the range of less than 10.

The first step comprises the steps of rotating the substrate, supplyingslurry having abrading particles dispersed to the surface of thesubstrate and pressing a tape to the surface of the substrate. The tapeto be used may be of a woven or non-woven cloth material. The first stepmay further include the step of washing the surface of the substrateafter the aforementioned approximately concentric circular marks havebeen formed.

The second step comprises the steps of rotating the substrate, supplyinga lubricant (not containing any abrading particles) to the surface ofthe substrate with the preliminary marks already formed thereon, andpressing a foamed tape on the surface of the substrate. The second stepmay further include the step of washing the surface of the substrateafter the aforementioned approximately concentric circular texturingmarks have been formed.

The foamed tape used in the second step comprises a base material formedin the shape of a tape and a foamed layer formed on the surface of thebase material. A plastic sheet with thickness in the range of 25 μm ormore and 125 μm or less may be used as the base material.

The foamed layer is characterized as having average diameter of airbubbles in the range of 1 μm or more and 50 μm or less, compressibilityin the range of 3% or more and 7% or less, compression recovery ratio inthe range of 40% or more and 60% or less, Shore D hardness in the rangeof 20 degrees or more and 30 degrees or less, and thickness in the rangeof 50 μm or more and 80 μm or less.

With a cleaning sheet of this invention, the surface area portion of thefoamed layer (exclusive of the air bubble portions) is large because theaverage diameter of the air bubbles inside is as small as in the rangeof 1 μm or more and 50 μm or less and preferably 30 μm or less. Sincethe compressibility of the foamed layer is in the range of 3% or moreand 7% or less, the foamed layer is compressed such that its surfacewill follow the shape of the surface of the workpiece when the surfaceof the foamed layer is pressed against the surface of the workpiece.Since the compression recovery ratio of the foamed layer is in the rangeof 40% or more and 60% or less, the surface of the foamed layer moves onthe surface of the workpiece such that the surface of the foamed layerfollows the surface of the workpiece as the foamed layer is movedrelative to the workpiece. In other words, the surface of the foamedlayer has a good characteristic of following the surface of theworkpiece. Since the Shore D hardness of the foamed layer is in therange of 20 degrees or more and 30 degrees or less, that is, since itshardness is sufficiently low, unwanted protrusions formed on the surfaceof the workpiece and foreign objects and dirt attached to the surface ofthe workpiece can be easily removed.

Because the foamed layer has such mechanical characteristics, theforeign objects and dirt attached to the surface of the substrate can beremoved in the second step without forming scratches on the surface ofthe substrate and without excessively scraping the surface of thesubstrate and indentations with an appropriate depth and protrusionswith an appropriate height can be formed such that the ratio Rmax/Ra isless than 10 and Ra is in the range 1Å or more and 6 Å or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a polishing machine which may use amethod of this invention.

FIG. 2 is a sectional view of a foamed tape used in the second step ofthe method of this invention.

FIG. 3 is a computer-generated surface image of the foamed layer of thefoamed tape of FIG. 2, obtained by a scanning electron microscope (SEM).

FIG. 4 is a computer-generated surface image of a substrate after thefirst step of the method of this invention (which is substantially thesame as a conventional texturing step), obtained by a scanning electronmicroscope.

FIG. 5 is a computer-generated surface image of a substrate after thesecond step of the method of this invention, obtained by a scanningelectron microscope.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a texturing method for forming texturing markson the surface of a magnetic hard disk substrate such as a glasssubstrate and an aluminum substrate.

A method of this invention may be carried out by using a double-sidepolishing machine as shown at 10 in FIG. 1 to form texturing marks onboth surfaces at the same time or by using a single-surface polishingmachine (not shown) of any kind known to persons skilled in the art toform texturing marks on only one surface at a time. Throughout herein“surface of a substrate” and “surfaces of a substrate” are both intendedto mean not only one of the surfaces of the substrate but also bothsurfaces of the substrate.

A texturing method of this invention comprises a first step of formingapproximately concentric circular preliminary marks on the surface of asubstrate and a second step of forming approximately concentric circulartexturing marks on the surface of the substrate based on theaforementioned preliminary marks such that the average surface roughnessof the surface of the substrate after the texturing marks are formed isin the range of 1 Å or more and 6 Å or less and that the ratio of themaximum surface roughness Rmax to the average surface roughness Ra onthe surface of the substrate with the texturing line marks formed is inthe range of 10 or less. The first step and the second step of themethod of this invention may be carried out by using the samedouble-side or single-side polishing machine or separately by usingdifferent polishing machines but it is preferable to carry them out byusing the same polishing machine.

With reference to FIG. 1, the first step comprises the steps of rotatingthe substrate 21 in the direction of arrow R, supplying slurry havingabrading particles dispersed to the surfaces of the substrate 21 throughnozzles 12 and pressing tapes 20 onto the surfaces of the substrate 21by means of contact rollers 11. The tapes 20 are delivered in thedirection of arrow T opposite to the direction R of rotation of thesubstrate 21. Approximately concentric circular line marks are thusformed on the surfaces of the substrate 21, say, as shown in FIG. 4.

After approximately concentric circular line marks are thus formed onthe surface of the substrate 21, a washing liquid such as water is blownonto the surfaces of the substrate 12 through nozzles 13 such thatforeign objects such as abrading particles remaining on the surfaces ofthe substrate 21 and polishing debris are removed. The removal of theseforeign objects is carried out by blowing the washing liquid onto thesurfaces of the substrate 12 while the substrate 12 is kept in therotating condition after the tapes 20 are separated from the surfaces ofthe substrate 21 and discharging these objects from the surfaces of thesubstrate 12 together with the washing liquid by utilizing thecentrifugal force of the rotating substrate 21.

The slurry is obtained by dispersing abrading particles in a dispersant.Particles of materials selected from alumina, ceria, silica and diamondare used as the abrading particles. The size of the abrading particlesis such that their average diameter is in the range of 0.02 μm or moreand 0.5 μm or less. A preferred example of the abrading particles iscohesive polycrystalline diamond particles which are secondary particleshaving diameters within the range of 20 nm or more and 150 nm or lesswith a plurality of polycrystalline diamond particles combined together,wherein the primary particles of these polycrystalline diamond particleshave diameters in the range of 30 nm or less and an average diameter inthe range of 4 nm or more and 10 nm or less. The content of abradingparticles in the slurry is 0.001 weight % or more and 0.5 weight % orless.

The dispersant comprises water and an additive, the additive includingat least two kinds selected from higher fatty acid amides, glycolcompounds, organic esters of phosphoric acid and surfactants. Thecontent of the additive with respect to the whole of the slurry is inthe range of 0.5 weight % or more and 5.0 weight % or less. Since nosignificant change is obtained on the surface of the substrate after thefirst step even if the slurry contains more than 5.0 weight % ofadditive, it is preferable to set the upper limit of the content at 5.0weight % in order to reduce the cost of the slurry.

Higher fatty acid amides function as a process accelerator forincreasing the processing speed of the first step. Examples of higherfatty acid amide that may be used include oleic acid diethanolamide,stearic acid diethanolamide, lauric acid diethanolamide, ricinolic aciddiethanolamide, ricinolic acid isopropanolamide, ersinic aciddiethanolamide, and tol fatty acid diethanolamide. Among these example,those with 12-22 carbon atoms are preferred. The content of higher fattyacid amide with respect to the whole of the additive is in the range of20 weight %-60 weight %. If the content is less than 20 weight %, theprocess speed becomes low. If it exceeds 60 weight %, abnormalprotrusions are generated.

Glycol compounds have affinity with abrading particles and function as adispersant. If a glycol compound is used when a dispersing medium isprepared, it serves to reduce the viscosity of the medium and hence amedium can be prepared more uniformly. Since they have affinity withwater, the substrate can be washed more effectively after the firststep. Examples of glycol compound that may be used include alkyleneglycol, polyethylene glycol, polypropylene glycol and diethylene glycolbutylether. The content of glycol compound with respect to the whole ofthe additive is in the range of 20 weight %-60 weight %. If it is lessthan 20 weight %, the dispersion characteristic of abrading particle isadversely affected such that it becomes easier for abrading particles tosink and large cohesive particles are formed. If it exceeds 60 weight %,it becomes hard to form clear texturing marks.

Organic esters of phosphoric acid have the function of inhibiting thegeneration of abnormal protrusions (burrs formed by polishing debrisattaching to the substrate surface) on the surface of the substrate.They are esters obtained by replacing hydrogen of phosphoric acid H₃PO₄with alkyl group or allyl group. Fatty acid salt type and aromatic salttype may be used. For example, phosphoric acid salt of polyoxyethylenenonylphenolether may be used. The content of organic ester of phosphoricacid with respect to the whole of the additive is in the range of 5weight %-40 weight %. If it is less than 5 weight %, abnormalprotrusions are generated. If it exceeds 40 weight %, it becomes hard toform clear texturing marks.

Surfactants have the effect of improving dispersing characteristic ofabrading particles. Surfactants of nonion or anion type can be used. Thecontent of surfactant with respect to the whole of the additive is inthe range of 20 weight % or less.

The slurry is obtained by adding abrading particles into water, furtheradding thereinto an additive including at least two agents selected fromhigher fatty acid amides, glycol compounds, organic esters of phosphoricacid and surfactants and mixing them by using a homo-mixer.

As the tape 20, a porous tape capable of acting elastically on thesubstrate surface and taking in foreign objects such as polishing debrisinside is used. A woven or non-woven cloth tape having at least itssurface portion made of fibers with diameter in the range of 0.1 μm ormore and 2.0 μm or less may be used as such a tape.

FIG. 4 shows approximately concentric circular line marks formed on asubstrate after the first step. Although no abnormal protrusions higherthan 10 nm are formed on the substrate surface, there are abnormalprotrusions with sectional shape of a pointed tower formed locally. Suchabnormal protrusions cast shadows at the time of forming a thin film,say, by sputtering, and cause spots on the formed film such as themagnetic layer. These abnormal protrusions can also cause collisionswith the magnetic head gliding above the substrate surface, adverselyaffecting the quality of recording and reproduction by the magnetic harddisk.

According to prior art technologies, a magnetic layer and a protectivelayer are formed on the substrate surface left with these line marksafter the first step by using a thin film technology such as sputtering.According to the present invention, on the other hand, the second stepis carried out to remove these abnormal protrusions such as spots havingill effects on the recording and reproduction by the magnetic hard disk,making the average surface roughness Ra in the range of 1 Å or more and6 Å or less and the ratio Rmax/Ra less than 10.

In other words, the second step of the texturing method according tothis invention is for removing unwanted protrusions, burrs, scratches,foreign objects and dirt remaining on the substrate surface after thefirst step such that texturing marks are formed on the substrate surfacemore accurately and this is done by trimming and cleaning the substratesurface. FIG. 5 shows the approximately circular texturing marks formedon the substrate surface after the second step. As shown, the abnormalprotrusions left on the substrate surface after the first step areremoved by the second step.

The same polishing machine used in the first step may be used for thesecond step or a different machine may be used for the purpose.Preferably, the polishing machine 10 used in the first step is also usedfor the second step. For the purpose, therefore, the tapes 20 (of wovenor non-woven cloth) used in the first step are removed after the firststep and replaced with tapes 30 of a foamed material.

With reference to FIG. 1, the second step comprises the steps ofrotating the substrate 21 in the direction shown by arrow R, supplying alubricant not containing any abrading particles to the surfaces of thesubstrate 21 having the aforementioned line marks formed thereon throughnozzles 14 and pressing the foamed tapes 30 onto the surfaces of thesubstrate 21 through the contact rollers 11. The foamed tapes 30 areadvanced in the direction shown by arrow T opposite the direction ofrotation R of the substrate 21. In this manner, the surfaces of thesubstrate 21 are trimmed and cleaned and texturing line marks asdescribed above are formed on them.

After the texturing marks are formed on the substrate surfaces based onthe aforementioned line marks, a washing liquid such as water is blownonto the surfaces of the substrate 12 though the nozzles 13 such thatforeign objects remaining on the surfaces are removed. The removal ofthese foreign objects is carried out by blowing the washing liquid ontothe surfaces of the substrate 12 while the substrate 12 is kept in therotating condition after the foamed tapes 30 are separated from thesurfaces of the substrate 21 and discharging the foreign objects fromthe surfaces of the substrate 12 together with the washing liquid byutilizing the centrifugal force of the rotating substrate 21.

Water or an aqueous solution may be used as the lubricant. The aqueoussolution is prepared by adding to water an additive that can reactchemically with the surfaces of the substrate. At least two kinds ofagents selected from higher fatty acid amides, higher fatty acids,metallic salts of higher fatty acids, glycol compounds and organicesters of phosphoric acid are added to water to produce such anadditive.

The amount of the additive to be added with respect to the whole of thelubricant is in the range of 0.5 weight % or more and 10 weight % orless. Since no significant change is found on the substrate surfacesafter the second step even if the additive is added to the lubricant inan amount in excess of 10 weight % and it only takes more time for thewashing in the second step, it is preferable to set the upper limit ofthe content to be 10 weight %.

The lubricant is alkaline, it being preferable that its pH value be pH7or over and pH12 or less. This is such that the surfaces of thesubstrate will not be overly scraped and that foreign objects such asparticles and oils attached to the substrate surfaces can be removed. Ifthe lubricant is acidic, the unevenness of the textured marks formed onthe surfaces of the substrate in the first step are excessively polishedand the texturing marks become unclear.

Higher fatty acid amides are used for removing the burrs and abnormalprotrusions remaining on the substrate surfaces after the first step.Examples of higher fatty acid amide that may be used include oleic aciddiethanolamide, stearic acid diethanolamide, lauric acid diethanolamide,ricinolic acid diethanolamide, ricinolic acid isopropanolamide, ersinicacid diethanolamide, and tol fatty acid diethanolamide. Among theseexample, those with 12-22 carbon atoms are preferred. The content ofhigher fatty acid amide with respect to the whole of the additive is inthe range of 10 weight % or more and 50 weight % or less. If the contentis less than 10 weight %, the removal of attached objects from thesubstrate surfaces become less effective. If it exceeds 50 weight %, thetexturing marks formed on the substrate surfaces are excessively scrapedand the marks become unclear.

Higher fatty acids and their metallic salts are effective agents forremoving foreign objects remaining attached to the substrate surfacesafter the first step. The metallic salts of higher fatty acids includesmetallic salts such as Na, K, Al and Ba of saturated or unsaturatedfatty acid. Examples of fatty acid include stearic acid, palmitic acid,myristic acid, oleic acid, lauric acid and behenic acid and those with12-22 carbon atoms are preferred. The content of salts of higher fattyacid with respect to the whole of the additive is in the range of 10weight % or more and 50 weight % or less. If it is less than 10 weight%, the removal of attached objects from the substrate surfaces becomeless effective. The removal characteristic is not much affected, on theother hand, if the content exceeds 50 weight %.

When higher fatty acid is added, alkanolamine is further added in orderto improve the affinity with water. The amount of alkanolamine to beadded with respect to the whole of the additive is in the range of 10weight % or more and 60 weight % or less. If it is less than 10 weight%, the lubricant may become white and opaque. Although it is increasedto more than 60 weight %, the affinity to water does not changesignificantly.

Glycol compounds serve to reduce the viscosity of the lubricant when itis prepared and makes it easier to wash the substrate after the firststep. Examples of glycol compound that may be used include alkyleneglycol, polyethylene glycol, polypropylene glycol and diethylene glycolbutylether. The content of glycol compound with respect to the whole ofthe additive is in the range of 5 weight %-50 weight %.

Organic esters of phosphoric acid have the function of inhibiting thegeneration of abnormal protrusions (burrs formed by polishing debrisattaching to the substrate surface) on the surface of the substrate.They are esters obtained by replacing hydrogen of phosphoric acid H₃PO₄with alkyl group or allyl group. Fatty acid salt type and aromatic salttype may be used. For example, phosphoric acid salt of polyoxyethylenenonylphenolether may be used. The content of organic ester of phosphoricacid with respect to the whole of the additive is in the range of 5weight %-40 weight %. If it is less than 5 weight %, abnormalprotrusions are generated. If it exceeds 40 weight %, the texturingmarks formed on the substrate surfaces in the first step are excessivelyscraped and their indentations and protrusions become unclear.

As shown in FIG. 2, the foamed tape 30 is comprised of a tape-shapedbase material 31 and a foamed layer 32 formed on the surface of thisbase material 31. FIG. 3 shows a computer-generated surface image ofthis foamed layer by a scanning electron microscope (SEM).

The thickness of the base material 31 is in the range of 25 μm or moreand 125 μm or less. The base material 31 is a plastic sheet with a flatand smooth surface and has a uniform thickness. A sheet made of asynthetic resin material such as polyester and. polyethyleneterephthalate (PET) is used as the plastic sheet.

The average air bubble diameter of the foamed layer is in the range of 1μm or more and 50 μm or less, and preferably in the range of 1 μm ormore and 30 μm or less.

Since the aforementioned second step is carried out as a wet process bysupplying a liquid lubricant between the surface of the foamed layer 32and the surface of the substrate 21, if the average air bubble diameteris less than 1 μm, the lubricity of the liquid between the surfaces ofthe foamed layer 32 and the substrate 21 becomes low and it becomesdifficult to take in the foreign objects removed from the surface of thesubstrate 21 into the interior of the foamed layer 32. If the averageair bubble diameter exceeds 50 μm, on the other hand, the surfaceportion of the foamed layer 32 (exclusive of the air bubble portions)acting on unit surface area of the substrate 21 such that not only doesit take longer for the trimming and cleaning of the surface of thesubstrate 21 but the lubricity of the liquid also becomes too largeinside the foamed layer 32 and between the surface of the foamed layer32 and the substrate 21 and the foreign objects once taken in areejected out and cause to scrape the surface of the substrate 21excessively or to form scratches on the surface of the substrate 21.

The compressibility of the foamed layer 32 is in the range of 3% or moreand 7% or less, the compressibility being defined as the change in thethickness of the foamed layer when the load thereon is 16 psi from thethickness at the time when the load therein is 1.4psi as measured underthe environmental condition of 23±3° C.

The surface of the foamed layer 32 becomes compressed as it is pressedagainst the surface of the substrate 21. If the compressibility of thefoamed layer 32 is less than 3%, it becomes difficult for the surface ofthe foamed layer 32 to be pressed by following the shape of the surfaceof the substrate 21 such that the compressive force of the surface part(exclusive of the air bubble portions) of the foamed layer 32 on thesurface of the substrate 21 becomes non-uniform and spots come to beformed on the finished or cleaned surface of the substrate 21. If thecompressibility of the foamed layer 32 exceeds 7%, on the other hand,the thickness of the foamed layer 32 becomes too small when the surfaceof the foamed layer 32 is pressed against the surface of the substrate21 and the volume for taking is liquid such as the cleaning liquidbecomes significantly reduced. As a result, the lubricity of the liquidsuch as the cleaning liquid inside the foamed layer 32 and between thesurface of the foamed layer 32 and the surface of the substrate 21becomes low and it becomes difficult to take in the foreign objectsscraped off the surface of the substrate into the interior of the foamedlayer 32.

The compression recovery ratio of the foamed layer 32 is in the range of40% or more and 60% or less, the compression recovery ratio beingobtained by measuring the displacement of the foamed layer 32 under aload of 12 psi under the environmental condition of 23±3° C. After theload is reduced to 1.6 psi, the recovered displacement in 30 seconds ismeasured and this measured displacement is divided by the aforementioneddisplacement at the time of the load of 16 pse, that is, the percentageratio of recovered displacement with respect to the compresseddisplacement.

If the compression recovery ratio of the foamed layer 32 is less than40%, the force of recovery by the compressed foamed layer 32 is too low,and the pressure of the surface portion (exclusive of the air bubbleportions) of the foamed layer 32 on the surface of the substrate becomeslow such that the force for removing the foreign objects attached to thesurface of the substrate 21 becomes low. If the compressive recoveryratio of the foamed layer 32 exceeds 60%, on the other hand, therecovery force of the compressed foamed layer 32 becomes too strong andthe pressure of the surface portion (exclusive of the air bubbleportions) of the foamed layer 32 on the surface of the substrate becomeshigh such that, as the objects scraped off the surface of the substrate21 are pressed onto the surface of the substrate 21, scratches areformed by them on the surface of the substrate 21.

The Shore D hardness of the foamed layer 32 is in the range of 20degrees or more and 30 degrees or less, the Shore D hardness being themeasured value under the environmental condition of 23±3° C. by using aShore D hardness meter according to JIS-L-1096.

If the Shore D hardness of the foamed layer 32 is less than 20 degrees,the force of removing foreign objects attached to the surface of thesubstrate 21 and the unwanted protrusions (abnormal protrusions) formedon the surface of the substrate 21 becomes low. If the Shore D hardnessof the foamed layer 32 exceeds 30 degrees, on the other hand, not onlythe unwanted protrusions (abnormal protrusions) formed on the surface ofthe substrate 21 but also necessary protrusions formed on the surface ofthe substrate 21 such as the protrusion parts of the texturing lines arescraped off, and it also becomes easier to form scratches on the surfaceof the substrate 21.

The thickness of the foamed layer 32 is in the range of 50 μm or moreand 800 μm or less. If the foamed layer 32 is too thin, lubricity of theliquid such as the cleaning liquid inside the foamed layer 23 andbetween the surface of the foamed layer 32 and the surface of thesubstrate 21 cannot be maintained at a high level and foreign objectscannot be effectively taken into the interior of the foamed layer 21 fora long time. If the foamed layer 32 is too thick, spots are generated asforeign objects attached to the surface of the substrate 21 and dirt areremoved. This is considered to be because the foamed layer 32 deformssignificantly in the direction of its surface during the trimming andcleaning processes and the geometrical structure of the foamed layer 32itself is significantly deformed.

The foamed tapes 30 are obtained by cutting a foamed sheet produced aswill be explained below into the form of a tape.

For producing the foamed sheet, a resin solution is mechanically stirredto obtain a paint having air bubbles with average diameter in the rangeof 1 μm or more and 50 μm or less (preferably in the range of 1 μm ormore and 30 μm or less) and foam magnification in the range of 2× ormore and 5× or less.

The resin solution is one containing urethane resin or acryl resin, andpreferably self-emulsifying aqueous urethane resin. In the above,aqueous urethane means waterborne polyurethane dispersion (WBPUD)obtained either by introducing into the main chain of polyurethane ahydrophilic component for dispersing stably in water or by dispersingwith an external emulsifier. Those obtained by the former method, or bydirectly introducing a hydrophilic component into the main chain ofpolyurethane, are referred to as self-emulsifying aqueous urethaneresin. (See, for example, “Recent Development in Technology of WaterbomePolyurethane Dispersion” by Toshifumi Tamaki, Dainippon Ink andChemicals, Inc.; http://www.dic.co.jp/rd/tech/rev0301/index.html). Ifself-emulsifying aqueous urethane is used, particles like aluminumhydroxide powder functioning as abrading particles need not be used. Inother words, hard particles which may become one of the causes forscraping the substrate surface excessively need not be used as externalemulsifier.

The resin solution may further contain an agent for accelerating thefoaming of this resin solution and for dispersing air bubbles stablyinside the paint. Such an agent is selected from higher fatty acids,denaturations of higher fatty acids and alkali salts of higher fattyacids. This agent is contained preferably at a rate of 30 weight partsor less as solid component and more preferably at a rate of 20 weightparts or less as solid component for 100 weight parts of resin solutionas solid component. If more than 30 weight parts as solid component arecontained, there is no significant change in the function ofaccelerating the foaming of the resin solution or dispersing air bubblesstably inside the paint. Higher fatty acid ammonium may be usedconveniently as an example of this agent.

The resin solution can be mechanically stirred by placing the resinsolution inside a container and rotating stirring vanes. For example, acontinuous high-pressure foaming machine (such as TW-70 (trade name)produced by Aikosha Seisakusho) may be used. The size of the air bubblesdispersed inside the paint and their foaming magnification can beadjusted by appropriately setting the rotational speed of the stirringvanes, the quantities of the resin solution and air and the time ofstirring.

Next, this paint is applied to the surface of the sheet-like basematerial to form a film comprising this paint on the surface of the basematerial. The application of the paint can be carried out by any of theknown coating methods such as the blade method, the gravier roll method,the knife method, the extrusion method, the reverse roll method and thecast method.

The coated film is dried next to form on the surface of the basematerial a foamed layer with average bubble diameter in the range of 1μm or more and 50 μm or less, compressibility in the range of 3% or moreand 7% or less, the compression recovery ratio in the range of 40% ormore and 60% or less and the Shore D hardness in the range of 20 degreesor more and 30 degrees or less.

The coated film is dried in an environment of 90° C.-160° C. In order tocompletely harden the coated film, far-infrared light may be used. Afoamed layer described above is thus formed.

The invention is described next by way of sample substrates of TestExamples 1-3 which were produced according to this invention by carryingout texturing process on substrates of magnetic hard disks which were2.5-inch aluminum substrate with the surface Ni—P plated andmirror-polished. These sample substrates were produced under the sameconditions except that the average diameter D50 of the abradingparticles in the slurry used in the first step were different.

The double-side polishing machine shown and described above was used forthe first step under conditions shown in Table 1. TABLE 1 First StepRotational speed of substrate 400 rpm Supply speed of tapes 60 mm/minuteSupply rate of slurry 15 ml/minute Hardness of contact rollers 40 duroOscillation frequency 5 Hz (amplitude = 1 mm) Compressive pressure ontapes 1.5 kg Time of processing 30 secondsAfter the first step, pure water was blown on the surface of eachsubstrate to wash it while the substrate was rotated. FIG. 4 shows thesurface condition of the substrate after the first step.

The composition of the slurry is shown in Table 2. The average diameter(D50) of the abrading particles was 0.05 μm for Test Example 1, 0.10 μmfor Test Example 2 and 0.15 μm for Test Example 3. TABLE 2 CompositionCohesive polycrystalline diamond 0.03 weight % of slurry particles(abrading particles) Pure water 94.97 weight % Additive 5 weight %Composition of Glycol compound 20 weight % additive (total = Ester ofphosphoric acid 40 weight % 100 weight %) Metal salt of higher fattyacid 40 weight %

The tapes were made of non-woven cloth of thickness 700 μm comprisingnylon fibers with thickness 1 μm.

The second step was carried out by replacing the tapes with foamed tapeson the same double-side polishing machine under the conditions shown inTable 3 after the first step was completed. After the time of processingmentioned in Table 3 has elapsed, pure water was blown onto the surfaceof the substrate for washing. FIG. 5 shows the surface condition of thesubstrate after the second step. TABLE 3 Second step Rotational speed ofsubstrate 800 rpm Supply speed of tapes 30 mm/minute Supply rate oflubricant 5 ml/minute Hardness of contact rollers 40 duro Oscillationfrequency 5 Hz (amplitude = 1 mm) Compressive pressure on tapes 0.5 kgTime of processing 5 seconds

The foamed tapes were obtained by cutting a foamed sheet which wasproduced as follows. First, a resin solution containing self-emulsifyingwaterborne polyurethane dispersion was prepared. When this resinsolution was prepared, an adjuster of foam formation and an adjuster ofbubble size and shape were added in order to accelerate the foaming ofthis resin solution and to disperse air bubbles stably inside the paint.The composition of this resin solution is shown in Table 4. The solidcomponent of this self-emulsifying waterborne polyurethane dispersionwas 40%. TABLE 4 Waterborne polyurethane dispersion 90 weight parts(self-emulsifying type): Product name: Superflex 410 Produced by:Daiichi Kogyo Seiyaku Kabushiki Kaisha Adjuster of foam formation:  4weight parts N-beef fat alkylsulpho-succinanamate/sodium sulfite Productname: FCU-305 Produced by: Sanko Kagaku Kogyo Kabushiki Kaisha Adjusterof bubble size and shape:  7 weight parts Higher aliphatic ammoniumProduct name: DC-100A Produced by: Sannopco Kabushiki Kaisha

Next, this resin solution was stirred by using a known type ofcontinuous foaming device (with the rotational speed of the rotaryvanes=2000 rpm) to produce a paint with foaming magnification 3x andhaving dispersed air bubbles with average diameter 30 μm.

Next, this paint was applied to the surface of a PET sheet of thickness50 μm by using a cylindrical blade coater of a known kind to form amembrane comprising this paint on the surface of this sheet. Thismembrane was completely dried in an environment of 100° C. to form afoaming layer of thickness 400 μm on the surface of the PET sheet toproduce a foamed sheet. Mechanical characteristics of the foamed layerof this foamed sheet are summarized in Table 5. TABLE 5 Average diameterof air bubbles 26μ Compressibility  5.3% Compression recovery ratio50.4% Shore D hardness 26 degrees

The composition of the lubricant that was used was as shown in FIG. 6.TABLE 6 Lubricant Pure water 95 weight % Additive  5 weight % AdditiveHigher fatty acid 35 weight % (total = 100 weight %) Glycol compound 30weight % Metallic salt of higher fatty acid  5 weight % Alkanol amine 30weight %

A comparison test was carried out by comparing the surface condition(the average roughness (Ra) and the maximum roughness (Rmax) of thesesubstrates after the texturing process with Comparison Examples forwhich texturing process was carried out according to prior arttechnologies.

In what follows, Comparison Examples will mean what were obtained onlyafter the first step in Test Examples. In other words, comparisons weremade between the surface conditions of the substrates after the firststep (Comparison Examples 1-3) and after the second step was donethereafter (Test Examples 1-3).

The average surface roughness Ra and the maximum roughness Rmax weremeasured by using a scanning electron microscope (Nanoscope Dimension3100 Series (trade name) produced by Digital Instruments, Inc.) Theresults of the comparison are shown in Tables 7 and 8. TABLE 7 Averagesurface Maximum surface Rmax/ roughness (Ra) roughness (Rmax) RaComparison Example 1 2.2 Å 30 Å 13.64 Comparison Example 2 4.5 Å 60 Å13.33 Comparison Example 3 5.3 Å 85 Å 16.04

TABLE 8 Average surface Maximum surface Rmax/ roughness (Ra) roughness(Rmax) Ra Test Example 1 2.0 Å 18 Å 9.00 Test Example 2 4.3 Å 41 Å 9.53Test Example 3 5.1 Å 50 Å 9.80

As shown in Tables 7 and 8, the ratio Rmax/Ra becomes less than 10 ifthe texturing process is carried out on the surface of the substrate bya method according to this invention, and it can be understood thattexturing marks having indentations with an appropriate depth andprotrusions with an appropriate height can be formed by a methodaccording to this invention.

1. A texturing method for forming texturing marks on the surface of asubstrate of a magnetic hard disk, said method comprising: a first stepof forming approximately concentric circular preliminary marks on thesurface of said substrate; and a second step of forming approximatelyconcentric circular texturing marks on said surface of said substratebased on said preliminary marks, wherein said surface after saidtexturing marks are formed in said second step has average surfaceroughness in the range of 1 Å or more and 6 Å or less and a ratio ofmaximum surface roughness to said average surface roughness in the rangeof less than 10; wherein said first step comprises the steps of rotatingsaid substrate, supplying slurry having abrading particles dispersed tothe surface of said substrate and pressing a woven or non-woven clothtape to the surface of said substrate; wherein said second stepcomprises the steps of rotating said substrate, supplying a lubricant tothe surface of said substrate with said preliminary marks formedthereon, and pressing a foamed tape on the surface of said substrate;wherein said foamed tape comprises a base material formed as a tape, anda foamed layer formed on the surface of said base material; and whereinsaid foamed layer has average diameter of air bubbles in the range of 1μm or more and 50 μm or less, compressibility in the range of 3% or moreand 7% or less, compression recovery ratio in the range of 40% or moreand 60% or less and Shore D hardness in the range of 20 degrees or moreand 30 degrees or less.
 2. The texturing method of claim 1 wherein saidfoamed layer has average diameter of air bubbles in the range of 1 μm ormore and 30 μm or less.
 3. The texturing method of claim 1 wherein saidfoamed layer comprises polyurethane resin.
 4. The texturing method ofclaim 1 wherein said first step further includes the step of washing thesurface of said substrate after the step of forming said approximatelyconcentric circular marks.
 5. The texturing method of claim 1 whereinsaid second step further includes the step of washing the surface ofsaid substrate after the step of forming said approximately concentriccircular texturing marks.